Optical coherence analysis relies on the use of the interference phenomena between a reference wave and an experimental wave or between two parts of an experimental wave to measure distances and thicknesses, and calculate indices of refraction of a sample. Optical Coherence Tomography (OCT) is one example technology that is used to perform high-resolution cross sectional imaging. It is often applied to imaging biological tissue structures, for example, on microscopic scales in real time. Optical waves are reflected from an object or sample and a computer produces images of cross sections or three-dimensional volume renderings of the sample by using information on how the waves are changed upon reflection.
There are a number of different classes of OCT, but Fourier domain OCT currently offers the best performance for many applications. Moreover, of the Fourier domain approaches, swept-source OCT has distinct advantages over techniques such as spectrum-encoded OCT because it has the capability of balanced and polarization diversity detection. It also has advantages for imaging in wavelength regions where inexpensive and fast detector arrays, which are typically required for spectrum-encoded OCT, are not available.
In swept source OCT, the spectral components are not encoded by spatial separation, but they are encoded in time. The spectrum is either filtered or generated in successive optical frequency sampling intervals and reconstructed before Fourier-transformation. Using the frequency scanning swept source, the optical configuration becomes less complex but the critical performance characteristics now reside in the source and especially its frequency sweep rate and tuning accuracy.
High speed frequency tuning, or high sweep rates, for OCT swept sources is especially relevant to in-vivo imaging where fast imaging reduces motion-induced artifacts and reduces the length of the patient procedure. It can also be used to improve resolution.
Traditionally, the swept sources for OCT systems have been tunable lasers. The advantages of tunable lasers include high spectral brightness and relatively simple optical designs. Another class of swept sources that has the potential to avoid inherent drawbacks of tunable lasers is filtered amplified spontaneous emission (ASE) sources that combine a broadband light source, typically a source that generates light by ASE, with tunable filters and amplifiers.
In order to compensate for instabilities and/or non-linearities in the tuning of the OCT swept sources, a sampling clock (k-clock) is often employed to enable sampling at equally spaced increments in the optical frequency domain (k-space). This k-clock must usually be delayed to match the delay associated with the optical signals in the sample and in the reference arms of the interferometer of the OCT system.